The latest generation of infrared long-baseline interferometric instruments combines high spatial resolution with spectroscopic
capabilities, enabling fascinating new studies of the AU-scale circumstellar environment around young stellar
objects. Here, we present recent investigations, which we conducted using the VLTI instruments AMBER and MIDI and
which demonstrate these new observational possibilities.
In one study, we combine near- and mid-infrared interferometry
(H-/K-/N-band) to constrain the geometry and radial
temperature profile of the circumstellar accretion disk around the Herbig Be star MWC147. Using detailed radiative
transfer modeling, we find strong evidence for the presence of an optically-thick inner gaseous disk. In another investigation,
we used AMBER's medium spectral resolution mode (R = 1500) to study the spatial origin of the hydrogen Brγ
line for five Herbig Ae/Be stars, associating the line emission with different physical mechanisms, such as disk winds and
magnetospheric accretion. Finally, we present AMBER H- and K-band observations of the close binary star θ1OrionisC and illustrate the benefits of fitting wavelength-differential visibilities and closure phases. Besides yielding a high observing
efficiency, this approach is also insensitive to calibration errors, induced, for instance, by fast changing atmospheric
conditions.